In regards to total hip arthroplasties Themistocles Gluck attempted the first hip femoral head arthroplasty with ivory implants in 1891 (Brand, Mont, & Manring, 2011). Since that time the total hip replacement has undergone a continually evolving spectrum of advancements in medical science. The evolution of the total hip replacement has led components to become stronger, more versatile, and wear resistant. Today's total hip replacements are said to potentially last indefinitely by their manufacturers. The total hip replacement has been at the center of medical technical advancements from perfecting the technique of hip surgical approaches to recent innovations in medications that reduce operative blood loss.
In 2006 the FDA approved the Birmingham Hip Resurfacing system (Canale, 2008). This new prosthesis allowed for a hip replacement without resection of the femoral neck. The femoral component allows for less bone resection than a total hip replacement. Many orthopedic surgeons began using this new innovative prosthesis until clinical studies showed an increase risk of femoral neck fracture in women (Powell, Belzile, Antoniou, et al., 2014). This is an important lesson that not all medical advancements are truly the next gold standard but each can provide an important step forward nonetheless, even if by trial and error.
The current standard total hip replacement is made from a chromium-cobalt alloy with two main components. The femoral component consists of a femoral intramedullary stem and neck with interchange head attachments. The femoral component has a spherical articulating head. The acetabular component is typically a metal alloy cup with a plastic polymer inner cup insert. The femoral component recreates the ball of the joint while the acetabular component recreates the cup.
The surgical procedure involved to insert the total hip replacement involves creating an osteotomy at the base of the femoral neck and reaming the intramedullary canal to implant the femoral component. The acetabular component is prepared by using sequential hemi-spherical reamers to prepare an acetabular bed that mimics the dimensions of the acetabular component.
A major source of focus of design modifications with the current total hip replacement is to minimize the rate of dislocation. Currently the dislocation rate in the US in regards to total hip replacements is 1-2% (Brown & Neumann, 2004). This is not a large number but for those patients unfortunate enough to experience a dislocation the repercussions can be devastating. Currently modifications to the total hip replacements include the use of large femoral heads, proper cup alignment, and thinner cup liner to accommodate the larger femoral heads. A potential solution for solving the problem of total hip prosthesis dislocation is to reverse the design of the components.
The concept of a reverse joint prosthesis has been previously described in patent US20070173945 by applicant Zimmer Technology, Inc. This design utilizes a reverse joint prosthesis of the shoulder. The shoulder joint prosthetic system shows a glenosphere component implanted into the humerus. This component then articulates with a humeral cup component which is implanted into the glenoid. This design is referred to currently as a reverse total shoulder prosthesis. The reverse total shoulder prosthesis is currently indicated for use to restore shoulder motion in patients with advanced rotation cuff disease (Drake, G., O'Connor, D., Edwards, T. (2010). Indications for Reverse Total Shoulder Arthroplasty in Rotator Cuff Disease. Journal of Clinical Orthopedics, 468(6)). This teaching of the shoulder joint prosthetic system makes no mention of the use of this prosthesis in the hip joint.
Implanting this device into the hip joint would most certainly fail for a multitude of reasons. The glenoid component is a stemmed component that utilizes a press-fit technique to gain bony purchase. This glenoid component could not be implanted into the acetabulum because of the limited bone stock in the medial wall of the acetabulum. The proximal humerus does not have a narrowing or neck below the articular surface. Therefore, this design would not be suitable for the hip because the femoral head has an anatomic neck which aligns the articular surface at 125 degrees from the shaft. This allows for stability through the joint with weight bearing.
The concept of a reverse total hip prosthesis has been described in publication US20110218637 by applicant Zafer Termanini in 2010. The patent design utilized an interlocking reverse hip prosthesis. A similar interlocking reverse total hip prosthesis was described in US20140025178 by applicant Hip Innovation Technology Llc in 2013. The interlocking design means that the acetabular and femoral components are joined together. The concept is beneficial because in theory this would eliminate the chance of dislocation. In reality the use of interlocking or constrained total hip prosthesis has widely fallen out of favor. The failure rate in long term follow up associated with constrained total hip prosthesis has been shown to be as high as 42 percent (Yang, Goodman, 2009). This failure is a result of loosening of the acetabular component which is ultimately pulled away from the pelvis secondary to the interlocking design (Yang, C., Goodman, S B. (2009). Outcome and Complications of Constrained Acetabular Components. Journal of Orthopedics, 32(2).).
A reverse total hip prosthesis was also described in U.S. Pat. No. 8,747,481 by applicant Brian Ted Maurer in 2012. This design did not include an interlocking system, though, this design has many practical limitations for clinical use. In the patent drawings, FIG. 12, elements 120 and 102 articulate with each other. In reality the design of the interaction between these two surfaces will entrap soft tissue and scar formation. Furthermore, this design is especially prone to acetabular loosening because element 118 of the FIG. 12 creates a lever arm with weight bearing on the acetabular component base. This post will create a rotational force on the acetabular base shearing it from the pelvis.